The present invention relates to a mass transfer process in which, in two or more contact stages, two or more liquid or gaseous phases are brought into contact with one another, one or more components being transferred between the phases and the phases moving countercurrently with respect to the overall configuration.
Mass transfer processes have already been in use for many years and in recent decades have undergone systematic consideration. In the present application, mass transfer processes are understood to mean processes in which at least one component is transferred from one phase to another phase. It relates to processes such as distillation, extraction and washing.
Depending on the manner of contact between the phases, mass transfer processes can be divided into the following classes:
Steady-state and non-steady-state processes. Steady-state processes are those processes in which the conditions, such as concentration at any location in the system, the volume of the different flows and the compositions of the different flows, are constant over time.
With regard to direction of flow of the phases through the process with respect to one another. In this way, processes are divided into concurrent, counter-current and crosscurrent.
The way in which the phases flow through the stages in which the contact between the phases takes place. In this context, the following distinction is made: continuous/continuous: both phases flow through the contact stages continuously; batch/batch: both phases flow through the contact stages in a batchwise manner; and continuous/batch: the flow through the contact stages is continuous for one phase and batchwise for the other phase.
In order to achieve efficient, large-scale mass transfer operations, the professional process engineering world nowadays primarily selects processes in which
the conditions are steady-state;
the phases move countercurrently in order to obtain the maximum number of equilibrium stages and thus the maximum level of mass transfer
both phases flow continuously through the contact stages.
These processes can therefore be referred to as steady-state, continuous/continuous countercurrent processes.
The general reasons cited by the professional process engineering world for selecting the steady-state design rather than the non-steady-state batch/batch design for liquid or gaseous phases when efficient mass transfer is desired are as follows:
Steady-state countercurrent processes make it possible, in equipment of identical size, to obtain a better separation of components than that which is achieved in non-steady-state process types.
To achieve the same separation level, choosing a steady-state counter-current process allows the equipment to be smaller than if a non-steady-state process type were to be selected.
In addition, processes with continuous/continuous flow are preferably selected owing to the fact that temporary storage is required for batch/batch processes.
These arguments in favour of selecting steady-state countercurrent processes are based on the fact that there is no knowledge, in the above-mentioned phase sector, of using non-steady-state countercurrent mass transfer processes with a plurality of stages. For this reason, comparisons are generally made between a single-stage non-steady-state process and a multistage steady-state countercurrent process. The result is what one might call an unfair comparison. This fact will be referred back to later.
An example of a steady-state continuous/continuous countercurrent distillation process is described in Dutch Patents 105668 and 105668. Although the descriptions of the processes given in these documents refer to a stepwise method, the phases are fed to and discharged from the contact stages continuously; both the liquid and the gaseous phases flow continuously through the contact stages.
An example of a steady-state continuous/continuous extraction process is described in U.S. Pat. No. 2,009,347. The device which is described in this document is designed in such a manner that each stage of the device comprises both a mixing zone and a separation zone. As soon as this process is fully operational, the feed and discharge of the heavy and light phases to and from each stage take place continuously.
An intermediate form between batch/batch processes and continuous/continuous processes is formed by the batch/continuous processes. In these processes, one of the phases flows through the contact stages in a batchwise manner and another phase flows through the contact stages continuously. These processes too can be of countercurrent design with more than one stage.
Since it is considered that steady-state continuous/continuous processes are the most efficient processes, non-steady-state, batch/continuous countercurrent processes with more than one stage have hitherto been designed only for solid/liquid extractions. The reason for this is that in processes of this nature there are practical drawbacks in carrying out a continuous/continuous steady-state process.
Thus far there is no knowledge of carrying out non-steady-state mass transfer processes between gaseous and liquid phases countercurrently, with the result that it has not been recognised that with a design of this nature the performance of the process is improved enormously if the phase with most back mixing flows in a batchwise manner.
It has now been found that it is also possible to carry out mass transfer processes in which the phases are liquid or gaseous under non-steady-state, countercurrent conditions. Therefore, the present invention provides a mass transfer process in which, in two or more contact stages, two or more liquid or gaseous phases are brought into contact with one another, one or more components being transferred between the phases and the phases moving countercurrently with respect to the overall configuration of the process, characterized in that the flow through one or more of the contact stages is batchwise for one phase and continuous for another phase.
According to the present invention, it has been found that it is possible to carry out countercurrent mass transfer processes with more than one stage under non-steady-state conditions. For example, if a distillation column is considered, the present invention offers the following advantages:
Owing to the lower strip factor (lower hydraulic load) in a batch/continuous design according to the invention, the diameter of a column can be reduced by up to 50% compared to the design which uses a continuous/continuous countercurrent process.
The power consumption of a batch/continuous process is lower by a factor of from 1.5 to 2 than that of a continuous/continuous design. In a batch/continuous process, the column height can be smaller by a factor of 2 than that of a continuous/continuous design.
Further advantages of the invention will be explained below with reference to specific embodiments.
The following definitions will be used in the present text:
A xe2x80x9csteady-state processxe2x80x9d is understood to mean a process in which the concentration at any location in the system, the volume of the different flows and the compositions of the different flows are independent of time.
A xe2x80x9cnon-steady-state processxe2x80x9d is understood to mean a process in which the parameters mentioned above are dependent on time.
xe2x80x9cContact stagexe2x80x9d is understood to mean that part of a process in which the phases are brought into contact with one another andxe2x80x94after mass transfer has taken placexe2x80x94are separated from one another again.
xe2x80x9cBatch(wise)xe2x80x9d flow through the contact stage is understood to mean that there is no significant feed or discharge of the relevant phase from and to the contact stage over a certain period of time.
xe2x80x9cContinuousxe2x80x9d flow through the contact stage is understood to mean that the relevant phase is fed and discharged continuously to and from the contact stage.
In this connection, it should be noted that if, with batch flow to a contact stage, so-called weeping or entrapment, for example, occurs, this flow is still considered to be batchwise.
In the case of continuous flow, the feed and discharge do not always have to be equal.
xe2x80x9cLiquidxe2x80x9d and xe2x80x9cgaseousxe2x80x9d phases, according to the invention, are also understood to encompass fluid phases such as those which occur in supercritical extraction. Such fluid phases have properties which are found both in liquid and in gaseous phases.
The present process can be employed, inter alia, in the following fields: distillation, extraction and washing processes, This means distillation, extraction and washing processes in a broad sense.
Distillation processes are to be understood as meaning all forms which occur in practice, such as stripping, absorption and rectification operations and reactive distillation and extractive distillation. In these processes, heat may be supplied or removed in the contact stages by means of external reboilers, coils arranged internally on the trays or other types of heat exchangers. It is also possible to combine the process according to the invention with, for example, packed beds in the distillation device, or alternatively combinations with new distillation techniques are possible. One example of such a new distillation technique is the partitioning of the distillation column in the horizontal direction, allowing parallel flows to be used in the distillation column.
The present invention is suitable in particular for the distillation of mixtures with a narrow boiling range. Such mixtures are known to the person skilled in the art. The following mixtures may be mentioned as examples: ethylene/ethane, propylene/propane, n-butane/i-butane and n-pentane/i-pentane. The invention is also particularly suitable for the cryogenic distillation of air/nitrogen mixtures.
Extraction and washing processes also refer to all such processes which are known in process engineering.
The present invention also includes mass transfer processes in which one of the phases is a liquid and another phase is a liquid which contains a dispersed solid, which phases transfer mass in the contact stage by mixing, and in which, after the mass transfer separation takes place at each contact stage resulting in liquid-containing solid phase and a liquid phase which does not contain any solid material. This process may be both a washing process and an extraction process.
The invention also encompasses couplings, combinations and applications in networks of process steps in which the process according to the invention, owing to its inherent advantage, itself provides an even greater advantage in the integrated process.
The invention is particularly suitable for processes in which the batch phase in the meantime undergoes another treatment. In this case, the batch phase may, if appropriate, be conveyed to a device situated outside the column. The desired treatment then also takes place in this device.
The invention also includes the use of hybrid processes in which the distillation, extraction or washing process in a device is coupled to other unit operations or separation stages, such as combinations with membrane, adsorption or crystallization separation. The use of the invention may also lead to better results in reaction steps in distillation, extraction or washing columns, and these also lie within the scope of the invention.
In the batch/continuous process, the heavy as well as the light phase may move through the contact stage in a batchwise manner. In the case of distillation, the heavy phase is the liquid or slurry, and in the case of extraction the heavy phase is the phase with the highest density.
It is preferable if the flow through two or more, and even more preferably through all, of the contact stages is batchwise for one phase and continuous for another phase.
In particular, the phases flow through contact stages which are formed by trays in a column, which trays are opened periodically so that the phase which flows through the column in a batchwise manner flows to the next tray.
It is also possible for the phases to flow through contact stages which are formed by separate vessels which are coupled in such a manner that, by diverting the phase which passes through the contact stages continuously, the batchwise phase can remain in the same vessel throughout the entire process, while the phases are nevertheless countercurrent. As well as separate vessels, it is also possible to use modified vessels in which separate zones are present.
Each contact stage may be coupled to a stage in which the two phases are separated from one another. This separator may be a gravitation settler, a centrifuge or a filtration device (including also membrane filtrations).
The invention also provides a device for mass transfer comprising
a reservoir with a top and a bottom;
a feed in the top and a discharge in the bottom, for feeding and discharging a phase;
a feed in the bottom and a discharge in the top, for feeding and discharging another phase;
two or more contact stages in which one phase is brought into contact with the other phase;
characterized in that
the contact stages are designed in such a manner that one phase flows through the contact stage in a batchwise manner and the other phase flows through the contact stage continuously.
The device described, which operates in accordance with this novel batch/continuous design, is superior to devices which are currently known in terms of both investment costs and operating costs, as well as performance.
In addition to the components described above, the device according to the invention may also include all standard components which are usually employed in continuous/continuous processes. Consideration may be given in this context to: additional means for feeding and discharging different flows, heat exchangers, etc.
If it is necessary, for certain applications, to damp pressure fluctuations caused by trays suddenly emptying, baffles or other types of internals may be installed on the tray in the descending liquid flow.
The contact stages in the device may be designed as trays which can rotate along a horizontal axis, in which case, when the tray is lying in the horizontal plane, the tray fits into the column in such a manner that the mass transfer can take place optimally, and, when the tray is rotated out of the horizontal plane, the desired phase can be periodically conveyed to the next tray.
The device may be designed in such a way that the phase which passes in a batchwise manner into successive trays in the column is transported by a device outside the column. If appropriate, it is also possible in this device to carry out a separate treatment of the phase which is passing through the device.
In addition to the phases between which the mass transfer takes place, other phases may also be present in the process. These other phases may be liquid, gaseous or solid. If there is a solid phase, this phase does not contribute to the mass transfer. This is the case, for example, if the phase to be extracted still contains solid particles, for example remainders of microorganisms. In this case, this solid material is dispersed in the phase to be extracted.
The present invention also provides a mass transfer process in which, in two or more contact stages, two or more liquid or gaseous phases are brought into contact with one another, one or more components being transferred between the phases and the phases moving countercurrently with respect to the overall configuration of the process, characterized in that the flow through two or more of the contact stages is batchwise for two phases.